The present invention relates to sensor systems and methods for determining the orientation of a satellite in orbit about earth or other celestial body. The invention relates more particularly to sensor systems and methods for determining the yaw orientation of a yaw-steered satellite about an axis pointing toward the center of the earth or other celestial body.
Optical sensors for satellites have been developed for viewing earth in order to derive position information of the satellite relative to earth. In such schemes, it is known to direct light from a field of view of the sensor onto a focal plane array, such as a charge coupled device (CCD), comprising a grid of pixels. The field of view and the optics of the sensor are typically designed such that at least part, and more typically all, of the circumference of the earth""s limb (i.e., the transition region between the earth and space) can be imaged onto the focal plane when the sensor is pointed in a suitable direction relative to the earth. The relative location of the image of the earth limb on the focal plane is determined by finding the pixels at which a large gradient in intensity of the incident light energy is located, a large gradient indicating a transition between earth and space. Using an appropriate algorithm, it is possible to determine the rotational orientation of the sensor, and hence of the satellite, about two orthogonal axes based on the locations of the transition pixels of the focal plane array. See, for example, U.S. Pat. No. 6,026,337.
A number of patents for various types of optical sensors have been acquired by the assignee of the present application, including U.S. Pat. Nos. 5,502,309, 5,534,697, 5,627,675, and 5,841,589, the entire disclosures of which are hereby incorporated herein by reference. The sensors described in all of the aforementioned patents have a single field of view for looking at the limb of the earth. As a rule, a sensor seeing the circular limb of the earth can be used for deriving position information about two axes, but cannot be relied upon for providing position information about the third axis. For example, if the sensor is looking along an axis that passes through the centroid of the earth, then any rotation of the sensor about that axis will not change the image of the earth limb on the focal plane array. This situation is not unlikely in many cases. For instance, some types of satellites are intentionally rotated about a yaw axis that is directed along the nadir vector through the centroid of the earth. As an example, yaw-steered satellites such as GPS satellites are deliberately yawed in order to position the solar panels of the satellite in an optimum position for receiving the sun""s radiation. If the optical sensor is mounted on the satellite so as to be looking along the yaw axis, then it is not possible for the sensor to provide information about the rotational position of the satellite about the yaw axis.
For this reason, on satellites using a limb-looking optical sensor as described above, it is necessary to derive the position information about the third axis by other means. One way to do this is to detect another celestial body with the sensor, such as stars or the sun. This solution, however is not entirely satisfactory. A star sensor requires an elaborate star map, and is generally not very accurate. A sun sensor only works when the sun is within view, such that another method for determining yaw orientation is required whenever the sun disappears behind the earth, or when the sun is in a location displaced a substantial amount from an orthogonal to the yaw axis.
U.S. Pat. No. 6,018,315 discloses a method and system for determining yaw orientation of a satellite using signals received from a global positioning system (GPS) satellite. A pair of spaced antennas are mounted on the satellite whose yaw orientation is to be deduced, and signals from the GPS satellite are received by both antennas and sum and difference signals are generated based on phase differences between the signals received by the antennas. The yaw angle of the subject satellite is correlated with the sum and difference signals such that yaw angle can be deduced from these signals.
One problem with using a method employing two antennas such as that of the ""315 patent is that the yaw angle solution can be multiple-valued. That is, the same sum and difference signals can be generated for two or more different yaw angles. For instance, if the pair of antennas is rotated about the yaw axis by 180 degrees, then each antenna will occupy the position previously occupied by the other antenna, and hence the same sum and difference signals will be generated by the antenna pair. This is not generally a problem if continuous tracking of the yaw orientation is performed, because the orientation of the satellite will be clear based on the previous yaw orientation history and the current sum and difference signals. However, if a computer upset should occur such that the yaw orientation must be established without benefit of knowledge of the previous yaw orientation history, then the multiple-valued nature of the yaw solution based on the sum and difference signals may make it impossible to deduce with certainty the yaw orientation of the satellite based solely on the sum and difference signals.
The present invention addresses the above needs by providing a method and apparatus for determining the yaw orientation of a satellite in which two (or more) separate pieces of information are generated for any given yaw orientation, and the combination of the two pieces of information is unique for each yaw orientation, thus avoiding the multiple-value problem. This is accomplished in accordance with the invention by providing two (or more) pairs of antennas. A first pair is spaced apart on the satellite along a first axis, such as the roll axis of the satellite. A second pair is spaced apart on the satellite along a second axis, such as the pitch axis of the satellite. Of course, the first and second axes can be oriented in any arbitrary sense relative to the satellite body axes as long as the orientation of the axes is known relative to the body axes, and as long as neither pair is aligned along the yaw axis. Each antenna pair is used to generate a difference signal by comparing the signals arriving at the two antennas. Orientation of the satellite about the yaw axis is deduced based on both difference signals.
In one embodiment of the invention, the difference signal derived for one antenna pair represents a phase angle difference between the signals arriving at the two antennas. The difference signal for the other antenna pair represents a signal strength difference derived by comparing the strengths of the signals arriving at the two antennas. In a preferred embodiment, each pair of antennas is used to derive both a phase angle difference and a signal strength difference. Using both the phase and signal strength difference, it is possible to deduce the yaw orientation of the satellite even when there has been a computer upset causing the yaw orientation history to be lost.
For example, if the first pair of antennas is aligned along a direction perpendicular to the direction along which a signal is arriving from the other satellite when such a computer upset occurs, it would be impossible to determine the correct yaw orientation based solely on the zero phase and strength difference signals that would be generated by the first antenna pair. The sensor system could determine that the first antenna pair is aligned perpendicular to the arrival direction of the signal, but there are two possible yaw orientations 180 degrees apart that satisfy that condition, and it would not be possible to determine which of those orientations is the true yaw orientation based only on the first antenna pair. However, in accordance with the present invention, the second antenna pair in this scenario provides a non-zero difference signal based on the strengths of the signals arriving at each antenna, and the sense of that difference is positive in one of the two possible yaw orientations and is negative in the other possible yaw orientation. Thus, the true yaw orientation of the satellite can be determined based on the information provided by both antenna pairs.
In a preferred embodiment of the invention, the satellite orientation about all three axes is determined by combining the antenna sensor system with an optical sensor arrangement that is operable to map a view of the celestial body onto at least one focal plane array and to determine orientations of the satellite about two body axes (e.g., the pitch and roll axes) thereof based on a location of a centroid of the earth or other celestial body relative to a center of the focal plane array. The optical sensor arrangement can be one such as described in commonly assigned U.S. patent application Ser. No. 09/756,395, filed Jan. 8, 2001, and entitled xe2x80x9cMethod and Sensor for Capturing Rate and Position and Stabilization of a Satellite Using At Least One Focal Planexe2x80x9d, the disclosure of which is hereby incorporated herein by reference.
Preferably, the antennas of the sensor system have directional sensitivity patterns such that the strength of the signal produced by each antenna is a function of the direction in which the received signal arrives at the antenna. In a preferred embodiment, the antennas of each pair are oriented such that minimum sensitivity occurs when signals arrive in the direction along which the two antennas are spaced apart and maximum sensitivity occurs when the signal arrives perpendicular to this direction. Preferably, one pair is spaced apart along the roll axis of the satellite and the other pair is spaced apart along the pitch axis.
The antennas are preferably mounted such that they all lie substantially in the same plane, which is normal to the yaw axis. This arrangement ensures that the difference signals are substantially unaffected by pitch and roll movements of the satellite.
The two pairs of antennas can comprise four antennas. Alternatively, however, one pair can comprise first and second antennas, and the other pair can comprise one of the first and second antennas together with a third antenna.
The sensor system can receive signals from more than one satellite in orbits in substantially the same orbital plane as the first satellite. For example, the first satellite can be one of a constellation of GPS satellites. The first satellite can receive signals from a satellite located ahead of the first satellite and from another satellite located behind the first satellite in orbit. An independent yaw orientation determination can be made based on each satellite""s signal, thus providing redundancy and improving reliability of the yaw determination.